FILE - In this Dec. 18, 2016, file photo, San Francisco 49ers quarterback Colin Kaepernick (7) and outside linebacker Eli Harold (58) kneel during the playing of the national anthem before an NFL football game against the Atlanta Falcons in Atlanta. (AP Photo/John Bazemore, File)

Colin Kaepernick’s effort to prove NFL team owners are conspiring to keep him off the field due to his protests of the national anthem will be difficult, but experts say it is growing increasingly clear that his behavior has discouraged teams from hiring him.

Attorney Mark Geragos has filed a grievance against NFL owners under their collective bargaining agreement, alleging that teams have colluded to “deprive Mr. Kaepernick of employment rights in retaliation for Mr. Kaepernick's leadership and advocacy for equality and social justice and his bringing awareness to peculiar institutions still undermining racial equality in the United States."

Kaepernick began sitting during the anthem, then later just kneeling, last August in an act of protest against racial inequality and police brutality. Though he is now a free agent, other players across the league have adopted the protest, drawing the ire of President Donald Trump, who claims the gesture is disrespectful to the military and says the NFL should not allow it.

In a statement, Geragos blasted team owners and Trump, claiming the president’s repeated complaints and threats over players kneeling during the national anthem have cowed the league into punishing players for peaceful protests. Owners are scheduled to meet this week to discuss a resolution to the anthem controversy.

Kaepernick opted not to return to the 49ers this season, reportedly out of concern that the team would cut him if he stayed. In March, sources close to the player indicated he would stand for the anthem if another team picked him up, but the damage to his reputation appears to be done.

“If not for Kaepernick’s protest, he would be on an NFL roster right now. Anyone who says otherwise is not being honest,” said Jeremi Duru, a professor at American University and co-author of “Sports Law and Regulation: Cases and Materials.”

Using a measure of adjusted net yards per attempt over the last three seasons, the Post found that Kaepernick’s performance is significantly better than three of them: Jared Goff of the L.A. Rams, Mike Glennon of the Chicago Bears, and rookie DeShone Kizer of the Cleveland Browns. Five others were nearly even with him, but 24 are outperforming him.

The data suggests that Kaepernick might not be in high demand even without his political baggage, but he was playing well enough to at least fit on somebody’s roster as a backup. Experts who study sports culture and law are confident there is more to Kaepernick’s continued unemployment than underwhelming performance on the field.

“By any football measure, Kaepernick should have a job in the NFL,” said Orin Starn, a professor of cultural anthropology at Duke University who teaches a course on sports and society. “He’s better than plenty of second-string quarterbacks in the league, and probably some starters. So there’s no doubt that his protest is the reason for no team having signed him.”

In the last couple of months, there have been instances where teams had openings for a player of Kaepernick’s caliber but appear to have shown no interest in hiring him. Instead, the Miami Dolphins brought in retiree Jay Cutler to replace injured Ryan Tannehill, and the Tennessee Titans signed Bandon Weeden as their backup quarterback.

“Even if we accept the idea that Kaepernick’s play has declined significantly in the last few years, and it has, he plays a premium position in a league where multiple teams have an obvious need and have chosen to sign quarterbacks with less talent,” said Dr. George McHendry, an assistant professor of communication studies at Creighton University.

Green Bay Packers starter Aaron Rodgers suffered a serious collarbone injury Sunday that will sideline him for most of the rest of the season, spurring new buzz about Kaepernick as a possible replacement.

“Of course there's reason to believe Colin Kaepernick is being blackballed,” said Stephen Mosher, coordinator of sports studies at Ithaca College. “He's a proficient quarterback. Green Bay certainly needs another QB and fast. Will he even get a look?”

Packers coach Mike McCarthy said after Sunday’s game that he currently plans to proceed with backup Brett Hundley starting and third-stringer Joe Callahan serving as his backup.

According to McHendry, the outsized attention on the protests Kaepernick started is the result of several factors.

“The anthem protests are a perfect storm for controversy,” he said. “They highlight lingering racial inequalities in America, they are happening on a massive scale (the NFL is big business), singing the national anthem is a revered performance of patriotism, and President Trump has drawn attention to the protests using divisive rhetoric which reframes the protest as about fealty to America.”

While Kaepernick’s activism might be holding back his career, the other players who have joined in his protests have not faced such drastic ramifications yet.

“Nobody else is facing anything close to the consequences Kaepernick is facing,” Duru said. “He was the first to protest inequities in the criminal justice system during the playing of the national anthem, and he has become the lightning rod.”

There are reports of backlash and boycotts, and owners like Jerry Jones of the Cowboys are now threatening to bench players who protest during the anthem, but nobody has lost their jobs over it. The meeting of team owners this week could result in a stricter policy, but anything seen as hampering players’ free speech would raise more complicated legal questions.

Marc Edelman, a professor at Baruch College who specializes in sports law, noted that Kaepernick’s troubles began when he left the 49ers after the 2016-17 season, but many of the players now protesting started during the current season, so the impact on their careers may not be clear for a few months.

“It will be interesting to see at the conclusion of this season what happens with the other players who are now kneeling,” he said.

There is a precedent for athletes’ careers being derailed by controversial political stances.

"Muhammad Ali, Tommie Smith, John Carlos, Marco Lokar, etcetera,” Mosher said. “History is full of athletes who paid the price for challenging the system.”

According to Starn, the Kaepernick case appears to be a particularly egregious example of an athlete suffering for their outspoken views, but the dynamic is nothing new.

“There has always been a risk to speaking up in American sports,” he said. “The sprinters John Carlos and Tommie Smith were sent back home after their famous black power protest in at the 1968 Olympics.”

Whether teams are rejecting Kaepernick because historically conservative owners disagree with him politically, because they fear their fans will be offended by his protests, or just because his public persona creates unneeded headaches, legal experts say proving that they colluded to do so is a steep hill to climb.

“I’m sure there is more to Colin Kaepernick’s situation than simply his on-the-field statistics, but the legal question is more nuanced than that,” Edelman said. “Any individual NFL team is allowed to reject a player for any reason or no reason at all, unless the underlying reason is an illegal one.”

If 32 separate owners decided Kaepernick was too distracting or too unpatriotic to put on the roster, and even if some did it after talking to President Trump, the player would have no recourse.

“Where you have a violation of the NFL collective bargaining agreement is if two or more teams come together and make a decision not to sign a player,” Edelman said.

Geragos will likely now seek documents and depositions from team and league executives in an effort to determine if such communications took place. If he manages to prove collusion in arbitration, Kaepernick could walk away with tens of millions of dollars in damages, but teams still would not have to hire him, which appears to be his ultimate desire.

"Colin Kaepernick's goal has always been, and remains, to simply be treated fairly by the league he performed at the highest level for and to return to the football playing field," Geragos said in his statement.

No matter how Kaepernick’s grievance with the league is resolved, Edelman suggested it may just be a precursor to an even bigger legal battle.

“It is at least theoretically possible we will see a claim brought by either a player or the union against the United States government,” he said. “The big unanswered question here is whether the activities of President Donald Trump in tweeting incessantly to encourage teams to sanction players in conjunction with his tweets threatening financial harm to NFL teams that do not comply amounts to public action by the U.S. government with the intent of quashing free speech.”

“It’s ironic that the NFL did its best to ignore domestic violence accusations against players, but had gotten so up in arms when one makes a peaceful silent protest before a game,” Starn said.

Domestic assault and drunk driving allegations did not inspire the president to declare players unpatriotic, threaten to change tax laws, or encourage his millions of Twitter followers to make their displeasure known to the league, though.

“I would argue that just about any transgression is okay with the NFL so long as the bottom line is secure,” Mosher said. “The oligarchs who run the league fear that perceived un-American acts by players will hurt them in the pocketbook.”

According to McHendry, the political priorities of professional sports leagues tend to follow those of the general public.

“Segregation in baseball was an extension of American racial politics,” he said. “The severity of the NFL’s view of illegal drugs is a reflection of the War on Drugs. The league’s long-running inconsistent and lax response to players accused of domestic violence connects to our cultural attitudes about violence against women.”

If Kaepernick is indeed being punished for his protests, it similarly reflects the polarized and partisan intensity that has overtaken the country.

“It is obvious that actions have consequences in the NFL,” McHendry said. “However, the severity and duration of those consequences say quite a bit about the politics of the league and where we are right now as a nation.”